It's the time of year when the bugs tend to come inside, looking for winter shelter. But what's happened to the Asian lady beetle, a common unwelcome houseguest in Minnesota? If it seems like fewer of the beetles are showing up in your home this winter, it's possible that things like changes in the beetles' food supply are behind it, said entomologists with the University of Minnesota Extension.
But at the same time, they warned a drop in population doesn't mean the beetles won't be back next year.
"There isn't really good data for Minnesota" on Asian beetle populations, said Bob Koch, an Extension entomologist in St. Paul. "The closest data we have is from a study in Michigan, which shows it could be an every other year cycle. It seems to be linked to the soybean aphid population."
Soybean aphids are the multicolored Asian lady beetle's favorite prey, Koch said, and when their numbers go up, more beetles survive and lay eggs. That in turn can mean more beetles finding their way inside houses to overwinter.
But in spring, Koch said, that same population explosion among the lady beetles means fewer aphids, and the beetles die off again for lack of food.
"It's a trend that's been seen in other predator and prey kinds of systems, too," Koch said.
Bruce Potter, an entomologist at the Extension's Southwest Research and Outreach Center, said other factors like weather conditions can also affect the number of soybean aphids in a given year, and in turn affect the lady beetles.
As its name implies, the Asian lady beetle is not native to North America. Extension reports say the beetles were first sighted in Minnesota in 1994, and have since spread throughout the state. Asian lady beetles look similar to other species of ladybugs, and have similar lifestyles, feeding on aphids and other insects. However, the Asian beetles tend to be bigger, can bite, and secrete a stinky liquid. When temperatures start to fall in the autumn, the beetles can congregate on houses or other buildings, trying to stay warm.
A dip in beetle populations doesn't necessarily mean the overall population is going down, Koch said. Invasive species like the Asian lady beetle tend to go through early population explosions and then settle into a more stable population, he said, but it will take more time for that trend to be documented.
Asian lady beetles have been disruptive to native lady beetle populations, Potter said, but their presence has some positive effects for Minnesota farmers and gardeners as pest control.
"The thing to remember is, as obnoxious as they are, they do serve a good purpose," Potter said. "If they were a little better behaved, that would be great."
For people still faced with swarms of lady beetles in their homes, the recommended method of removing them is with a nylon stocking placed over the hose attachment of a vacuum cleaner, Koch said. The beetles are trapped in the stocking, and prevent the vacuum cleaner from picking up the bugs' odor.